1. Field of the Invention
This invention relates generally to elastomers and more specifically, to elastomers useful for inner liners of pneumatic tires.
2. Description of the Related Art
Various articles are constructed to hold a gas, such as air, under pressure. These articles include, for instance, tires, athletic balls such as basketballs, footballs, etc., inflatable boats, air mattresses, and the like. These articles are typically made from a polymeric material having some elastic properties. Tires, for instance, are typically made from an elastomeric rubber material, such as a styrene butadiene polymer.
Many elastomeric materials that are used to make inflatable articles may, in some circumstances, remain slightly permeable to gases such as oxygen. If left unchecked, the gas permeability of the inflated article may compromise the performance of the article and cause the article to deflate over time. Further, oxygen passing through the article can cause oxidation of the elastomers, causing deleterious effects to the properties of the elastomer. For instance, the elastomers may tend to harden and degrade.
In view of the above, inflatable articles, such as tires, typically contain an inner liner that is intended to reduce gas permeability and inhibit oxygen travel through the article. For instance, in the past, these inner liners have been made from a composition containing butyl rubber. Butyl rubber in its raw state, however, still remains somewhat gas permeable. Thus, many attempts have been made to combine butyl rubber with other materials in order to further reduce permeability. For example, attempts to improve the gas permeability of butyl rubber have involved adding a filler to the rubber material.
Unfortunately, great amounts of fillers need to be added to butyl rubber in order to more than minimally improve the gas permeability of the material. Adding relatively large amounts of fillers, however, may adversely affect the mechanical properties of the rubber. For instance, the rubber material may tend to be too brittle, non-flexible, and have reduced cohesion. The above effects not only adversely affect the performance of the liner material, but also make it very difficult to process the material.
In order to improve the mechanical properties of a butyl rubber and filler mixture, attempts have been made to add a plasticizer to the formulation. Plasticizers, such as oils and waxes, however, typically adversely affect the permeability properties of the rubber composition reversing the effects of adding any fillers.
As such, a need currently exists for an improved composition that may act as a barrier layer in inflatable articles, such as tires. More particularly, a need exists for a rubber composition that may be used as a barrier layer that not only has relatively low gas permeability, but that can also be easily processed by having a relatively low modulus and viscosity.